Tight-binding Linear Muffin-tin Orbital Research Articles

Structural, magnetic and hyperfine parameters of B2‐type FeAl compounds doped with transition metal additions

AbstractThe structural, electronic and magnetic properties of the transition metal (TM = Ti, V, Cr, Mn, Co, Ni, Cu, Zn) alloying additions in FeAl compound with B2 structure have been investigated with tight‐binding linear muffin‐tin orbital (TB‐LMTO) method. The site preference of the TM additions was investigated for (Fe0.44TM0.06)Al0.50 and Fe0.50(Al0.44 TM0.06) compositions. Both magnetically ordered and non magnetic solutions were analysed. The analysis has shown that the elements (Ti, V, Cr, Mn) slightly more prefer the Al sublattice, whereas the (Co, Ni, Cu, Zn) atoms have a distinct preference for the Fe sublattice. The local magnetic moment of the additions located at the Al sites shows an antiferromagnetic alignment with respect to the bulk magnetization. The dependence of the isomer shift and hyperfine magnetic field for 57Fe nucleus on the TM valence is analysed. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

STUDY OF THE ELECTRONIC STRUCTURE AND PHYSICAL PROPERTIES OF THE IRIDIUM BASED INTERMETALLIC COMPOUNDS UNDER PRESSURE

The electronic structure of the Iridium based L12 intermetallic compounds ( A 3 B ) such as Ir 3 Ti , Ir 3 Zr , Ir 3 Hf , Ir 3 V , Ir 3 Nb and Ir 3 Ta , which have wide applications as high temperature structural materials are studied by means of Self-Consistent Tight Binding Linear Muffin Tin Orbital (TB-LMTO) method. These compounds are found to crystallize in the Cu 3 Au type structure. The total energies are calculated as a function of volume and fitted to Birch equation of state to find the equilibrium lattice parameter and the bulk modulus. They are tabulated and compared with the available experimental and other theoretical data. The partial number of electrons at A and B sites of these compounds are calculated as a function of volume. We find that there is a continuous transfer of d-electrons from A-site to B-site. The band structure and density of states histograms are plotted. From the DOS histograms, we find that the plots are similar for all the compounds except for Ir 3 V . In the case of Ir 3 V , we find that there are hybridization between Ir -d like and V -d like states at the Fermi level. Hence, it is predicted that under compression there may be a Lifshitz type of transition in Ir 3 V . The cohesive energy, heat of formation and the electronic specific heat coefficient of the compounds are also computed.

Electronic structure and structural phase stability of CuAlX2 (X=S, Se, Te) under pressure

The electronic and structural properties of chalcopyrite compounds CuAlX2 (X=S, Se, Te) have been studied using the first principle self-consistent Tight Binding Linear Muffin-Tin Orbital (TBLMTO) method within the local density approximation. The present study deals with the ground state properties, structural phase transition, equations of state and pressure dependence of band gap of CuAlX2 (S, Se, Te) compounds. Electronic structure and hence total energies of these compounds have been computed as a function of reduced volume. The calculated lattice parameters are in good agreement with the available experimental results. At high pressures, structural phase transition from bct structure (chalcopyrite) to cubic structure (rock salt) is observed. The pressure induced structural phase transitions for CuAlS2, CuAlSe2, and CuAlTe2 are observed at 18.01, 14.4 and 8.29 GPa, respectively. Band structures at normal as well as for high-pressure phases have been calculated. The energy band gaps for the above compounds have been calculated as a function of pressure, which indicates the metallic character of these compounds at high-pressure fcc phase. There is a large downshift in band gaps due to hybridatization of the noble-metal d levels with p levels of the other atoms.

Chemisorption of Co monolayer on H-passivated Si(1 1 1) surface: Comparison with clean Si(1 1 1) surface

The chemisorption of Co monolayer on H-passivated Si(1 1 1) surface was studied theoretically using tight binding linear muffin-tin orbital (TB-LMTO) method and compared with that of the clean Si(1 1 1) surface. For the clean Si(1 1 1) surface, Co atoms prefer to adsorb on the interstitial center site and might even sit below the Si surface, forming a Co–Si mixed layer at the interface. On the contrary, for the H-passivated Si(1 1 1) surface, the energetically favorable position for the Co adsorption is at the top site and Co atoms cannot adsorb below the H layer or diffuse into the lattice. The present results indicate that the H passivation layer effectively hinders intermixing between Co and Si atoms.

Electronic structures of LaNiIn and LaNiInH x ( [formula omitted], 1 and [formula omitted

First-principle studies of the electronic structure of LaNiInH x for x = 0 , 1 3 , 2 3 , 1 and 4 3 are performed using the spin-polarized tight binding linear muffin-tin orbital (TBLMTO) method. These compounds crystallize in the ZrNiAl-type crystal structure. For LaNiIn, a gap occurs (between − 5.0 and 4.2 eV), below which the s-states of In and Ni atoms are present. In the energy range from − 4.2 eV to E F , there is a large number of electronic states with mainly La 5d, Ni 3d and In 5p character. The doped hydrogen atoms change the electronic structure. Low concentration of hydrogen ( x ≤ 2 3 ) gives the additional H 1s band in the lower part of the valence band in the − 4.0 and − 6.0 eV region which destroys the energy gap. With increasing hydrogen content for x = 1 , 4 3 the H 1s band broadens ( − 1.5 to − 8 eV). The calculations of the density of states at the Fermi level show that the studied materials have metallic character.

Study of electronic structure and elastic properties of transition metal and actinide carbides

We study the systematics of the electronic structures of the carbides of the transition series Sc, Ti, V, Zr and Hf and the actinides U and Th using the tight-binding linear muffin-tin orbitals method (TB-LMTO). The densities of states, charge distribution and energy bands are studied in the lowest energy configuration. Changes in the bonding configurations and the equations of state are also compared as we go from 3d Ti to 5d–4f Hf. For the actinide carbides, the f-states have features rather different from the 3d–5d transition metal carbides. Similar studies are made on these carbides.

First-principles electronic structure calculations of [formula omitted] (R being the rare-earth elements Ce–Lu)

The paramagnetic electronic structure calculations of the R 3 Al 5 O 12 ( R = Ce – Lu , the rare-earth elements) systems by substituting Y atoms in Y 3 Al 5 O 12 ( YAG ) with R atoms, using the tight-binding linearized muffin-tin orbital (TB-LMTO) method within local density approximation (LDA) have been reported. Our calculated band structure and density of states (DOS) for the doped systems clearly show that the ground state of the R = Ce – Yb systems are metallic while the R = Lu system (it has completely filled 4f states) is insulating. This insulating phase of Lu 3 Al 5 O 12 is in very good agreement with the experimental results. Our calculations correctly brings out the narrow bandwidth feature of 4f states. Our studies clearly show that the position of the O-2s/p in YAG remains intact while substitution, the O-2p and R-4f energy gap remains almost the same up to near half filling of 4f states ( R = Ce – Pm ) , from then it decreases continuously up to R = Yb and finally it becomes zero for R = Lu (insulator) due to a strong hybridization between the O-2p and Lu-4f states. Our calculated cohesive energy ( E coh ) per molecule for these systems decreases from R = Ce – Sm (up to half filling of 4f bands) then increases from R = Sm – Tb , decreases from R = Tb – Yb (more than half filling) and finally increases for R = Lu , which is an insulator. This decreasing and increasing trend of E coh is exactly similar to that of the rare-earth elemental solids, indicating that the rare-earth elements in the YAG system plays crucial role in deciding the physical properties of the doped systems. However, there are no experimental results available to our knowledge to compare our results.

Magnetism of the Fe9 nanocluster supported on Ni(0 0 1)

The morphology and the spin-polarized electronic structure of the Fe9 cluster supported on the (001) surface of Ni are investigated by combining the Modified Embedded Atom Model (MEAM) with quenched Molecular Dynamics (MD) and a selfconsistent non-collinear spd Tight-Binding (TB) method parameterized to ab-initio Tight-Binding Linear Muffin-Tin Orbital (TB-LMTO) results. Two-dimensional geometry and overall ferromagnetic coupling are found, in keeping with recent experimental findings. The local magnetic moments and Fe–Ni hybridization effects are analyzed in terms of the interfacial relaxation.

Multiple collinear magnetic arrangements in thin Mn films supported on Fe(001). Antiferromagnetic versus ferromagnetic behavior

We present a theoretical study of the magnetic properties of thin Mn films of 6 and 7 monolayers supported on Fe(001). The ab-initio tight binding linear muffin tin orbital (TB-LMTO) method was used to investigate the competition between ferromagnetic (F) and antiferromagnetic (AF) couplings within the system. We found several collinear magnetic solutions that may coexist at room temperature. The most stable configurations are characterized by AF coupling between the surface and subsurface Mn layers together with F coupling between Mn and Fe at the interface. The ground state arrangements for the 6 and 7 Mn films display opposite sign of the surface magnetic moment relative to the Fe substrate. The implications of these results in the possible onset of non-collinear magnetism when a step is present at the interface are discussed in comparison with Cr/Fe systems where non-collinear magnetism has been recently reported.

Electronic structure and structural stability of ThGa2 under pressure: an ab initio study

The high structural stability of ThGa2 that exists under pressure is investigated by band structure calculations as a function of reduced volume using the tight-binding linear muffin-tin orbital (TB-LMTO) method. Our calculations indicate that the Fermi level sits in a pseudogap and does not move even under large compressions. The band dispersion curves also show very small changes under compression indicating that transfer of electrons from ‘s’-like to ‘d’-like bands does not occur. The above features may be the reason for the wide stability regime of ThGa2 under pressure. The behaviour is unique when compared with that of other known rare-earth and actinide based di-gallides.

Theoretical investigation of the electronic structure and structural phase stability of CeGa 2 under pressure

Recently, Chandra Shekar et al. (Phys. Stat. Sol. B 241(2004)2893), studied the structural stability of CeGa 2 under high pressure up to ∼32 GPa and reported a structural transition from hexagonal AlB 2-type to omega trigonal-type starting at ∼16 GPa with a volume collapse of ∼6%. The high-pressure omega triginal phase is found to coexist with the parent phase up to 32 GPa. In this paper, we report the results of our band structure calculations on this system as a function of reduced volume by the tight-binding linear muffin–tin orbital (TB-LMTO) method, in order to look into this structural transition and to understand it in terms of changes in its electronic structure. Our calculations indicate a structural transition at ∼30.6 GPa with a volume collapse of 3.5%, in good agreement with the experimental results. The possible mechanism of the phase transition may be due to f→d electron transfer under pressure. The theoretically calculated ground-state properties, namely the lattice parameters and the bulk modulus are also in good agreement with the experimental values.

Nanogranular Phase Formation During Devitrification of Fe(NiCo)ZrB Amorphous Alloys

A comprehensive review of our recent experimental and theoretical developments in the processing of nanocrystalline soft magnetic materials made by crystallization of amorphous precursors and containing new nanocrystalline phases is given. The relationship between the structures of the metastable and equilibrium phases and their transformations are discussed. Nickel-rich amorphous precursors with stoichiometry Ni64Fe16Zr7B12Au1 were produced by melt-spinning technique and then heat-treated at temperatures ranged from 420 °C to 600 °C for one hour to form nanostructured alloy. The transformation from the amorphous state into the nanocrystalline state was investigated by the differential scanning calorimetry (DSC), the x-ray diffraction (XRD), the vibrating sample magnetometry (VSM) and Mossbauer techniques. The annealing favours the emergence of cubic FexNi23-xB6 crystalline grains (10-25 nm in diameter). Magnetic measurements made at 4.2-1100 K reveal rather high value of saturation magnetization (nearly 60 and 40 Am2/kg at 4.2 K and room temperature, respectively) in amorphous as well as in nanocrystalline states. These facts are consistent with 300 K 57Fe Mossbauer results which are well supported by the calculations of Ni and Fe magnetic moments in Ni23B6 and Fe23B6 phases, using the spin polarized tight binding linear muffin-tin orbital (TB-LMTO) method. However, anomalous high magnetic moments of Fe and Co atoms were found in some inequivalent positions in the crystal structure.

Influence of local environment on Co magnetic behavior in Y 2(Co,M) 7B 3 compounds, where M = Cu or V

The magnetic properties of Y 2Co 7− x M x B 3 compounds, with M = Cu or V, and x ≤ 2, were investigated in the temperature range 4.2–850 K and in magnetic fields of up to 7 T. Electronic structure calculations using the tight binding linear muffin-tin orbitals method in the atomic sphere approximation (TB-LMTO-ASA) were performed for the compounds with x = 1. The variations of the Curie temperatures, spontaneous magnetization and of the mean Co moment at 4.2 K are discussed in correlation with the structural characteristics, site localization of the M atoms and dilution effects. The Co behavior is described in the general theory of spin fluctuations.

Study of [formula omitted]-Al 2O 3 and the role of Y in YAlO 3 and Y 3Al 5O 12 by first principles electronic structure calculations

The ground state electronic properties of the α -Al 2O 3, YAlO 3 and Y 3Al 5O 12 (YAG garnet) systems have been studied using the tight-binding linearized muffin-tin orbital (TB-LMTO) method. We have also performed studies of the three compounds under isotropic compressions and expansions. For YAlO 3, we have varied a, b and c lattice parameters to get the ground state. The calculated equilibrium lattice parameters for all these systems and the energy gap for Al 2O 3 agree very well with experimental and earlier theoretical results. We have calculated the cohesive energy and bulk modulus for them and a comparison with the earlier results (available only for Al 2O 3 and YAlO 3) shows good agreement. Our calculations correctly account for the insulating property of these systems. The gap is found to be direct for Al 2O 3 and YAG and indirect for YAlO 3. Moreover, the band gap values are found to be decreasing from Al 2O 3 to YAlO 3 to Y 3Al 5O 12, perhaps due to the Y atom as well as due to the increased number of atoms per primitive cell. The band gap for all these systems increases monotonically with pressure, suggesting that the gap may due to a strong Al/Y s/p-O p hybridization in these systems. Our calculated density of states (DOS) for Al 2O 3 compares very well with the theoretical and experimental results and for the rest, experiments are yet to come to confirm our results. The calculated P–V relation for Al 2O 3 compares very well with earlier calculations; however, it deviates slightly for YAlO 3 due to the difference in the calculated equilibrium lattice parameters from the experimental values.

Ab initio Study of the Electronic Structure of Rhodium Based Intermetallic Compounds under Pressure

AbstractFor Abstract see ChemInform Abstract in Full Text.

Investigation of the electronic structure and chemical bonding of lead hexacyanoferrate(III)

The electronic structure of lead hexacyanoferrate(III) is calculated by the ab initio tight-binding linear muffin-tin orbital (TB-LMTO) method in the LSDA + U approximation. The influence of vacancies in the lead sublattice on the electronic spectrum, chemical bonding, and magnetic properties of the Pb1.5Fe(CN)6 phase is investigated. Analysis of the electronic spectrum shows that this compound is characterized by semiconductor conductivity. It is demonstrated that the semiconductor gap is associated with the charge ordering of iron(III) ions.

Electronic and structural properties of CuMO 2 (M = Al, Ga, In)

The electronic and structural properties of delafossite transparent conducting oxides, namely CuMO 2 (M = Al, Ga, In), have been studied by means of self-consistent Tight Binding Linear Muffin-Tin Orbital (TB-LMTO) method. Electronic structure and hence total energies of these compounds have been computed as a function of reduced volumes and fitted with Birch Murnaghan equation. The calculated equilibrium lattice parameters and bulk modulus are in good agreement with experimental and reported values. The energy band gap of 2H (hexagonal) and 3R (trigonal) CuMO 2 (M = Al, Ga, In) has been calculated and compared with available data. The stability of 2H and 3R polytypes has also been studied.

X-ray photoemission spectra and electronic band structure of the ternary compounds U 3M 2M 3′, M = Al, Ga, M′ = Si, Ge

The magnetically ordered compounds U 3M 2M 3′ where M = Al, Ga and M′ = Si, Ge were investigated by computing their electronic band structure and measuring the X-ray photoemission spectra (XPS). The band structure was calculated by the tight-binding linear muffin-tin orbital (TB-LMTO) method in atomic spheres approximation (ASA). The XPS was measured in vacuum (∼5 × 10 −10 Torr) on freshly cleaved surfaces of the samples. The valence band spectra agree rather well with the measured ones. The satellite structure of the deep U(4f) lines was recovered and discussed.

Stability and magnetism of Ni–Fe alloyed overlayers on Fe(001)

The surface ordered alloy formation of Ni/Fe(001) overlayer system is explored, using the tight-binding linear muffin-tin orbital (TB-LMTO) method. The alloy formation for one- and two- surface layer of Ni–Fe on Fe(001) are considered by calculating the total and surface formation energies for different configurations of Ni overlayers on Fe(001). The preferred ground states for different configurations are determined and Ni (alloyed with Fe) is found to remain at the Fe surface. The magnetic profile for these configurations is found comparable to the experimental predictions.

Ab initio study of the electronic structure of rhodium based intermetallic compounds under pressure

The electronic structure of rhodium based L12 intermetallic compounds (A3B) such as Rh3Ti, Rh3Zr, Rh3Hf, Rh3V, Rh3Nb, and Rh3Ti, which find wide applications as high temperature materials are studied by means of the self-consistent tight binding linear muffin tin orbital (TB-LMTO) method. These compounds are found to crystallize in the Cu3Au type structure. The total energies are calculated as a function of volume and fitted to Birch equation of states to find the equilibrium lattice parameter and the bulk modulus. They are tabulated and compared with the available experimental data. The partial number of electrons at A- and B-site of these compounds are calculated as a function of volume. We find that there is a continuous transfer of d-electrons from the A-site to the B-site. The band structure and density of states histograms are plotted. From the density of states (DOS) histograms we find a double peak structure near the Fermi level in the case of the Rh3V, Rh3Nb, and Rh3Ta compounds whereas this kind of structure is absent in the Rh3Ti, Rh3Zr, and Rh3Hf compounds. The cohesive energy and the electronic specific heat coefficient of the compounds are also computed.